As the recording density of a magnetic disk increases, the thermostability of the remanent magnetization of bits recorded in the magnetic disk is reduced. For this reason, a ferromagnetic material improving the intensity of an anisotropy magnetic field is used for a recording layer, whereby the thermostability of the remanent magnetization is improved. Meanwhile, if the ferromagnetic material improving the intensity of the anisotropy magnetic field is used, an increase in the maximum recording magnetic field that a magnetic head can generate cannot keep up, so that the magnetic field is insufficient for magnetic recording.
In order to compensate the insufficiency of the magnetic field necessary for magnetic recording, a thermally assisted magnetic recording system has been proposed. In the thermally assisted magnetic recording system, at the time of recording, a laser beam is irradiated onto a magnetic disk so as to locally heat the magnetic disk, such that the strength of a reversal magnetic field is reduced. In this thermally assisted magnetic recording system, in order to stabilize the quality of a signal, it is important to evaluate a variation of a coercive force at the same temperature in the recording unit of the magnetic disk.